The invention relates to aeronautics, but more particularly, the invention relates to lift modifications of sustaining airfoils (wings) that intrinsically combine to safely reduce takeoff and landing air speeds of a T-tail aircraft having a stick shaker/pusher activated by an angle of attack rate of change sensor.
An aircraft wing is shaped to provide a required lift with a least possible drag. However, the shape of the wing is compromised between that aerodynamic shape that is most efficient for aircraft cruising speed and that aerodynamic shape required for low air speeds such as encountered during takeoff and landing. Typically, high speed (M .gtoreq. .6) wings are thin and require extensive movable surfaces that effect wing reshaping for low airspeeds (M .ltoreq. .15).
As noted in the publication, Hickey et al, Large-Scale Wind-Tunnel Tests of an airplane model with a 45.degree. sweptback wing of aspect ratio 2.8 Employing High-Velocity Blowing over the Leading-and Trailing-Edge Flaps (NACA RM A58A09):
"the use of thin, low-aspect ratio, sweptback wings on modern aircraft seriously limits the low-speed maximum lift and longitudinal stability."
To compromise the difference between high and low speed lift and drag, many wings are provided with various movable control surfaces such as trailing edge flaps or leading edge devices that are normally held in a passive position at cruising speed but are extended for low airspeeds.
In the past, movable leading edge devices have been preferred over fixed wing leading edges because they effect efficient leading edge reshaping for high and slow speed flight performance. When a high speed wing is operated at slow speeds, it is subject to leading edge stall because the airflow over the wing must turn abruptly in order to flow up and around the leading edge. As it turns, the airflow may separate from the upper wing surface, thereby initiating wing stall. To delay airflow separation, leading edge devices are typically used which extend forward and downward from the wing. The movable leading edge devices allow the wing to achieve higher angles of attack before it stalls. The leading edge devices effectively reduce the minimum airspeed of a high speed wing. In other words, the movable leading edge devices reduce the stall speed of the wing without hindering the thin profile required for high speed performance, e.g., Mach .gtoreq. .6.
While movable leading edge devices enhance the performance of a high speed wing, they require auxiliary structure, power and controls for moving them from a passive to active position; this adds weight and cost and further equipment complication which can reduce reliability.
A widely used contour modification for increasing the maximum lift coefficient of thin sweptback wings is to droop the wing leading edge with or without an increased leading edge radius. Anderson et al., A Flight Investigation of the Effect of Leading-Edge Camber on the Aerodynamic Characteristics of a Swept-Wing Airplane, NACA RM A52L16a, 1953; Demele et al, The Effects of Increasing the Leading-Edge Radius and Adding Forward Camber on the Aerodynamic Characteristics of a Wing with 35.degree. Sweepback, NACA RM A50K28a, 1951; Dew, Effects of Double-Slotted Flaps and Leading Edge Modifications on the Low-Speed Characteristics of a Large-Scale 45.degree. Sweptback Wing with the without Camber and Twist, NACA RM A51 D18, 1951; Evans, Leading-Edge Contours for thin Swept-Wings: An analysis of Low-and High-Speed Data, NACA RM A57B11, 1957 (declassified 1959); Goradia et al, Laminar Stall Prediction and Estimation of C.sub.L.sbsb.(max), Journal of Aircraft, vol. 11, no. 9, Sept. 1974, pp. 528-536; Hicks et al, Effects of Forward Contour Modification on the Aerodynamic Characteristics of the NACA 64, -212 Airfoil Section, NASA TM X-3293, September 1975; Kelly, Effects of Modifications to the Lead-Edge Region on the Stalling Characteristics of the NACA 63, -012 Airfoil Section, NACA TN 2228, 1950; Maki, Full-Scale Wind-Tunnel Investigations of the Effects of Wing Modifications and Horizontal-Tail Location on the Low-Speed Static Longitudinal Characteristics of a 35.degree. Swept-Wing Airplane, NACA RM A52B05, 1952; Maki, An Investigation of Subsonic Speed of Several Modifications to the Leading-Edge Region of the NACA 64 A010 Airfoil Section, NACA TN 3871, 1956.
While it is generally known that thin wing performance can be improved somewhat at low speed by increasing the leading edge radius and drooping the leading edge such as illustrated at page 27 of the Maki (1952) reference supra, the prior art leading edge radius changes do not effect improved performance over all airspeeds. Moreover, prior art leading edge radius changes are restrictive as to the location of radius change for given wing types. For example, the Evans reference supra, concludes at page 8 that improved wing performance at low speeds can be achieved by increasing the leading edge radius at the outboard portions of the wing and that a full span radius contour is likely to result in wave drag penalty which inhibits performance. The findings of the references are quite restrictive in that there are other secondary factors which may affect performance such as; wing sweep; wing taper; wing aspect ratio; or other surfaces attached to the wing which influence aerodynamic performance such as wing tip tanks. However, there is one thing in common in the references and that is that the leading edge radius is defined as a fraction of chord length. This is the standard practice as originally developed with the NACA airfoil section definitions. The NACA practice is to define thickness ordinates as a fraction of the airfoil chord. The basic chord is defined in unitless form so that families or airfoils may be easily extrapolated therefrom. Commonly, a wing definition is linearly interpolated between the wing root and the wing tip at some predetermined ratio making the chords at the root and tips proportional. Accordingly, the radius at the leading edge of the wing changes dimension from a larger value at the root to a smaller value at the tip.
A wing to which this invention is particularly directed is derived from the NACA 64A 109 airfoil section: 6 designates the NACA series; 4 designates a chord-wise position of minimum pressure expressed in tenths of the chord; "A" designates an airfoil that is substantially straight from about eight tenths of the chord to the trailing edge; 1 designates a design lift coefficient in tenths; and the 09 expresses the thickness of the wing as a percent of chord. A wing with this particular airfoil type was tested with an associated aircraft to evaluate their aerodynamic characteristics. The results of this test are given in the publication: Soderman, Full-Scale Wind-Tunnel Test of a small unpowered jet aircraft with a T-tail, NASA TN D 6573, 1971. Several leading edge configurations forward of the 6 percent chord line of the standard wing were tested and analyzed. Of these, one was a blunt leading edge with a leading edge radius that varied linearly from root to tip and another was a 30.degree. drooped leading edge that simulated a hinged-type leading edge. The blunt and drooped configurations were tested for their effectiveness at slow airspeeds. While the blunt leading edge configuration marginally reduced wing stall speed, the 30.degree. drooped leading edge significantly reduced wing stall speed. A hinged system would be required for the 30.degree. droop to maintain high speed performance whereas the blunt configuration would only marginally decrease high speed performance.
To further evaluate leading edge modifications to the NACA 64A 109 airfoil, a Model 36 Gates Learjet aircraft was configured with a "blunt-droop" leading edge forward of the 6 percent chord line over approximately the outboard 40 percent of the free wing semi-span as suggested by Evans, page 8 (1957) supra, for the purpose of improving stall speed characteristics of the T-tail aircraft. The leading edge had a 1.3 inch radius throughout the 40 percent outboard portion. It was concluded that this leading edge modification only marginally improved stall speed (i.e., reduced stall speed approximately 3 knots and that stall characteristics were essentially unchanged. Accordingly, this leading edge configuration did not yield the predicted performance increase as suggested by the above references.
Other passive devices may be used to enhance wing performance at low speeds. For example, for a high speed wing with tip tanks, airflow around the tip may be undesirably misdirected. The airflow misdirection can cause the tip portion of the wing to stall permaturely. Airflow over the tip portion of the wing can be substantially redirected with strakes that extend forward of the leading edge to intersect the tip tank. An example of such a strake appears at page 63 of the publication Aviation Week and Space Technology, Nov. 4, 1968. A detailed discussion of the influence of the strake appears in the February 1969 issue of the magazine "Flying" in a pilot report by Mack Miller. A photograph of a flying Learjet aircraft with strakes appears on the cover of the August 1969 issue of "Contrails."
Aircraft may exhibit unfavorable stall characteristics, such as unpredictable roll in either direction. Comparatively, a desirable stall is one in which there is a smooth pitch down of the aircraft without roll.
To improve safety, aircraft with undersirable characteristics are typically equipped with a stick shaker/pusher system that is activated by an angle of attack sensor. When a selected angle of attack is reached, the shaker/pusher system is activated which first shakes the aircraft's control column and then at a lower air speed pushes it forward to effect aircraft nose down pitch. The sensor and shaker/pusher combine to provide an artificial stall with desirable characteristics. The shaker/pusher system is set to actuate at an airspeed sufficiently above true stall to provide the desired characteristics for all types of stall entry. The shaker/pusher system (e.g., maximum limit of angle of attack) must be set to activate at higher airspeeds for high entry rates (e.g., 3 - 5 knots/second deceleration) even through the setting could be at a lower air-speed for entry rates of 1 knot/second as established by the FAA for the definition of stall speed. As a result, the artificial stall initiated by the shaker/pusher system will be above the true stall speed (e.g., 3 - 5 knots above) for normal entry rates.
Some aircraft wing retrofiters have employed stall turbulators or strips at the wing leading edge to initiate stall over the inboard section of the wing while premitting the outboard section of the wing to remain flying. While such a configuration may effect a premature stall with improved stall characteristics in some situations, it does not eliminate the requirement for a stick shaker/pusher system. The stall turbulators at the wing leading edge to not come without penalty. They may increase the minimum stall speed while also increasing drag at high speed. An example of a wing stall turbulator is disclosed in the publications: SAE Paper 760471 and "Mark II System", Dee Howard Company (1975).
A stall turbulator and angle of attack sensor may collectively combine to give a pilot a feeling of satisfactory stall characteristics and raise the minimum airspeed of such T-tail aircraft. This is because the stall turbulators may increase the minimum stall speed and the angle of attack sensor must be set to accommodate all reasonably predictable rates of change of pitch attitude. The effects of the stall turbulator and differential velocity are mathematically combined to raise minimum airspeed of such T-tail aircraft. The leading edge of this invention in combination with a T-tail aircraft, a stick shaker/pusher and an angle of attack rate of change sensor overcome these difficulties.